To avoid web tracking and web ads, it is often useful to blacklist domains in the HOSTS file. On Windows XP, the HOSTS file is drive:\WINDOWS\system32\drivers\etc.
Make a backup before editing the file. To avoid breaking the earlier entries, add new entries to the bottom of the file.
An entry to black hole a domain will look like the following.
This shows before and after for a host lookup of cdn.optmd.com. In the second ping, it resolves to the localhost.